Near infrared doped phosphors having an alkaline gallate matrix

ABSTRACT

Phosphors based on doping of an activator (an emitter) into a host matrix are disclosed herein. Such phosphors include alkaline gallate phosphors doped with Cr 3+  or Ni 2+  ions, which in some embodiments can exhibit persistent infrared phosphorescence for as long as  200  hours. Such phosphors can be used, for example, as components of a luminescent paint.

The present application claims the benefit of U.S. ProvisionalApplication No. 61/244,258, filed Sep. 21, 2009, which is incorporateherein by reference in its entirety.

GOVERNMENT FUNDING

The present invention was made with government support under Grant No.ONR N00014-07-1-0060 from the Office of Naval Research. The Governmenthas certain rights in this invention.

BACKGROUND

Phosphorescence is a phenomenon that the light emitted by a phosphorlasts after stoppage of excitation for duration of time sufficient forlight to be perceived by the eye or a detection system, i.e., 0.1 secondor longer. Phosphorescence that lasts for several hours at roomtemperature is called long-persistent phosphorescence. A phosphor thathas long-persistent phosphorescence is called a long-persistentphosphor, or a long-lasting phosphor, or a long-afterglow phosphor.

Persistent phosphorescence was discovered in the 11^(th) century inChina and Japan and in the 16^(th) century in Europe. In persistentphosphors, two kinds of active centers are involved: emitters and traps.Emitters are centers capable of emitting radiation after being excited.Traps do not emit radiation, but store excitation energy by trappingelectrons and holes and release it gradually to the emitter due tothermal stimulation. Emitters are usually a small amount ofintentionally added impurity atoms or ions. Co-activators are oftenintentionally added to form new trapping centers to improve thepersistence time and intensity of the phosphors.

The importance of persistent phosphorescence was recognized since 1960s,and various persistent phosphors in the visible spectrum have beendeveloped since then. Known in the art of long-persistent phosphors aresulfides, aluminates, and silicates.

The first generation long-persistent phosphors, sulfides [such as ZnS:Cu(green), CaS:Bi (blue), and CaS:Eu,Tm (red)] have been practically usedfor several decades. The disadvantages for sulfide phosphors includeshort persistence duration (e.g., three hours at the longest) andinstability when ultraviolet light and moisture coexist. For thesereasons, the sulfides have found only limited applications such as inluminous watch and night-time display inside a house.

Recently, aluminate-based long-persistent phosphors attractedconsiderable attention because of their better chemical stability,higher brightness, and longer persistence time (e.g., up to 20 hours)compared to the sulfide-based phosphors. Aluminate-based long-persistentphosphors are available in green and blue regions. The popular greenaluminate phosphors include SrAl₂O₄:Eu²⁺ and SrAl₂O₄:Eu²⁺,Dy³⁺. Knownblue aluminate persistent phosphors include CaAl₂O₄:Eu²⁺,Nd³⁺ andSrAl₄O₇:Eu²⁺,Pr³⁺/Dy³⁺. The main drawback of these alkaline earthaluminates is that when they contact with moisture and water, hydrolysisreaction occurs quickly, which limits the out-door applications of thesephosphors.

Another popular long-persistent phosphor is silicates, which arepotential alternatives for the aluminates. The silicate-based phosphorsinclude (Sr_(2-x),Ca_(x))MgSi₂O₇:Eu²⁺,Dy³⁺ with emission tunable fromcyan to blue, green, and to yellow; Ca₃MgSi₂O₈:Eu²⁺,Dy³⁺ with afterglowband at 475 nm; MgSiO₃:Eu²⁺,Dy³⁺,Mn²⁺ with emission at 660 nm; andCa_(0.2)Zn_(0.9)Mg_(0.9)Si₂O₆:Eu²⁺,Dy³⁺,Mn²⁺ with emission at 690 nm.

From the above list, it can be seen that all the persistent phosphorsdeveloped up to now are in the visible region. The longest wavelength isin red at 690 nm (Ca_(0.2)Zn_(0.9)Mg_(0.9)Si₂O₆:Eu²⁺,Dy³⁺,Mn²⁺). Some ofthese visible persistent phosphors (such as SrAl₂O₄:Eu²⁺,Dy³⁺) have beencommercialized and widely used for security signs, emergency signs,safety indication, indicators of control panels, and detection of highenergy rays, and so on. In contrast, no persistent phosphors in theinfrared or near infrared region are available in market.

Infrared or near infrared long-persistent phosphors have gainedconsiderable attention in recent years because of strong military andsecurity demands. For surveillance in night or dark environments,infrared or near infrared emitting taggants are generally used fortagging, tracking, and locating the targets of interest. For practicalmilitary and security applications, it is desirable for the taggants topossess one or more of the following characteristics. (1) The emissionfrom the taggants should be in infrared or near infrared spectrum, whichis invisible to naked eyes but is detectable to specific infrareddetection devices (such as night vision goggles) from far distance. (2)The infrared or near infrared emission from the taggants should bepersistent for more than 10 hours (overnight) without additionalcharging (excitation). Ideally, the taggants can be repeatedly chargedby solar radiation in daytime. (3) The taggants should be stable enoughto withstand various out-door application environments includingapplications in water. (4) The taggants should be able to be insertedalmost anywhere, including into liquid solution, dyes, paints, inks,epoxies, and sol-gel, which can then be coated onto almost any surfacefor concealment. (5) The production of the phosphors should be easy andcheap. Unfortunately, up to now, no such infrared or near infraredtaggants have been available.

In design of infrared or near infrared phosphors, transition metalchromium in trivalent state (i.e., Cr³⁺) and nickel in divalent state(Ni²⁺) were widely used as the luminescent centers. Chromium can emit anarrow luminescence band around 696 nm due to the transition of ²E→⁴A₂,or a wide band in the near infrared region related to the transition of⁴T₂→⁴A₂, which strongly depend on the crystalline field strength of thehost. When crystal field is strong, the first excited state will be ²Eterm and causes luminescence properties of the materials like in Al₂O₃(ruby). In weak crystal field, ⁴T₂ term will become lowest excited stateand causes broad band emission like in BeAl₂O₄ (alexandrite). Since the²E→⁴A₂ transition is a spin-forbidden transition, the lifetime is of theorder of milliseconds. On the other hand, the lifetime of wide bandemission, which is spin-allowed, is around microseconds. Nickel has acomplicated emission spectrum due to the appearance of emissiontransitions from more than one level. The emission spectra of Ni²⁺ inthe octahedral site for garnets such as Y₃Al₅O₁₂ and Gd₃Sc₂Ga₃O₁₂consists of three bands in near infrared due to ³T₂→³A₂ transition. Atroom temperature, the bands are broad with a maximum at 1360 nm inY₃Al₅O₁₂, 1450 nm in Gd₃Sc₂Ga₃O₁₂, and 1200 nm in MgAl₂O₄.

It has been reported that some Cr³⁺ doped gallates showed strongemission in the infrared. The reported gallates includeLa₃Ga₅GeO₁₄:Cr³⁺, La₃Ga₅SiO₁₄:Cr³⁺, Li(Ga,Al)₅O₈:Cr³⁺, and MgGa₂O₄:Cr³⁺.But no afterglow phenomenon was reported.

SUMMARY

In one aspect, the present disclosure provides a phosphor including amaterial having one or more of the following formulas: AGa₅O₈:xC, yR;and AGaO₂: xC, yR, wherein a portion of Ga may optionally be replacedwith a Group IIIA metal (e.g., B, Al, and/or In) and/or a Group IVAmetal (e.g., Ge, Si, and/or Sn); and wherein each A is independently analkaline metal (e.g., Li, Na, and/or K); each C is independently Cr³⁺,Ni²⁺, or a combination thereof; each R is independently a Zn²⁺ ion, analkaline earth metal ion (e.g., Mg²⁺, Ca²⁺, Sr²⁺, and/or Ba²⁺), or acombination thereof; each x is independently 0.01 to 5 and representsmol % based on the total moles of Ga and any replacements thereof; andeach y is independently 0 to 5 and represents mol % based on the totalmoles of Ga and any replacements thereof. In some preferred embodiments,each x is independently 0.05 to 0.5 and represents mol % based on thetotal moles of Ga and any replacements thereof In other preferredembodiments, each y is independently 0.1 to 2 and represents mol % basedon the total moles of Ga and any replacements thereof. For embodimentsin which C is Cr³⁺, the phosphor can have emission band peaks at 690 to1100 nm. For embodiments in which C is Ni²⁺, the phosphor can haveemission band peaks at 1100 to 1550 nm. For embodiments in which C is acombination of Cr³⁺ and Ni²⁺, the phosphor can have emission band peaksat 690 to 1100 nm and 1100 to 1550 nm.

In certain embodiments, a phosphor as disclosed herein can be capable ofbeing activated by one or more of solar radiation, ultraviolet lamp,fluorescent lamp, and light emitting diode (LED) light. In someembodiments, the solar radiation includes diffused light and directsunlight in an outdoor environment (e.g., a sunny day, a cloudy day, ora rainy day) that may include an open area, a shadow of a tree, or ashadow of a building. In preferred embodiments, the phosphor is capableof being activated for a time between sunrise and sunset. In preferredembodiments, the phosphors disclosed herein can be quickly charged bysolar radiation, ultraviolet light, and fluorescent lamp light: e.g.,less than 1 minute of excitation can result in up to 200 hours ofcontinuous near infrared emission.

In certain embodiments, a phosphor as disclosed herein can be capable ofbeing activated in one or more of air, tap water, salt water, seawater,bleach water, and bleach-salt-sodium bicarbonate (NaHCO₃) aqueoussolution. In preferred embodiments, a phosphor as disclosed herein ischemically stable in one or more of air, tap water, salt water,seawater, bleach water, and bleach-salt-sodium bicarbonate (NaHCO₃)aqueous solution.

In preferred embodiments, an emission from an activated phosphor asdisclosed herein can persist for up to 200 hours after excitation.

A phosphor as disclosed herein can be in the form of, for example, apowder (e.g., typically a white powder), a ceramic, or nanoparticles.For embodiments in which the phosphor is a powder, the powder can bemixed with water-based or oil-based paints to form a luminescent paint.Water-based luminescent paints include, for example, general indoor-useswall paints. Oil-based luminescent paints include, for example,oil-based resins (e.g., epoxy resins, polyurethane resins, polyesterresins, acrylic acid resins, and/or hydroxyl acrylic acid resins) and/orvarnishes (e.g., amino varnishes, acrylic polyurethane coatings, and/ortransparent alkyd coatings). Such luminescent paints can be capable ofbeing activated by, for example, one or more of solar radiation, anultraviolet lamp, and a fluorescent lamp, and can have emission bandpeaks at 690 to 1100 nm for Cr³⁺-activated phosphors and/or 1100 to 1550nm for Ni²⁺-activated phosphors. In preferred embodiments, the emissioncan persist for up to 200 hours after excitation. Thus, the presentdisclose provides luminescent paints that include phosphors as disclosedherein, which can provide the paints or inks with the function of nearinfrared luminescence in the dark.

The present disclosure also provides method of activating a phosphor.Such methods include providing a phosphor as disclosed herein andexposing the phosphor to one or more of solar radiation, an ultravioletlamp, a fluorescent lamp, and a light emitting diode (LED) light.

The present disclosure also provides activated phosphors. The activatedphosphors include a phosphor as disclosed herein that has been exposedto one or more of solar radiation, an ultraviolet lamp, a fluorescentlamp, and a light emitting diode (LED) light. In preferred embodiments,the activated phosphor has an emission that persists for up to 200 hoursafter excitation.

The phosphors disclosed herein can be used in a variety of applications,e.g., in luminous paints, as near infrared lighting sources, and fornight vision devices and manufactures.

Definitions

As used herein, “a,” “an,” “the,” and “at least one” are usedinterchangeably and mean one or more than one.

As used herein, the term “comprising,” which is synonymous with“including” or “containing,” is inclusive, open-ended, and does notexclude additional unrecited elements or method steps.

In traditional definition, the persistence of phosphorescence of visiblephosphors is measured as persistence time which is the time, afterdiscontinuing irradiation, that it takes for phosphorescence of a sampleto decrease to the threshold of eye sensitivity. See, for example, U.S.Pat. No. 6,953,536 B2 (Yen et al.). This threshold is the signal levelof emission intensity that a naked eye can clearly see in the dark. Forinfrared phosphors, however, this definition is no longer valid becausethe infrared signal is invisible to unaided eye. The persistence timefor infrared phosphors should then be determined by the sensitivity ofthe detection systems such as nigh vision goggles, infrared cameras, orinfrared detectors. As used herein, the persistence time of infraredphosphors is the time that it takes for an eye can see with the aid of aGeneration III night vision goggle in a dark room. In addition, thedecay of the phosphorescence intensity is measured by aFluoroLog3-2iHR320 spectrofluorometer.

The above brief description of various embodiments of the presentinvention is not intended to describe each embodiment or everyimplementation of the present invention. Rather, a more completeunderstanding of the invention will become apparent and appreciated byreference to the following description and claims in view of theaccompanying drawings. Further, it is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the excitation and emission of exemplary LiGa₅O₈:0.001Cr³⁺phosphors where the excitation spectrum was monitored at 717 nm.

FIG. 2 shows the excitation and emission of exemplary LiGa₅O₈:0.001Cr³⁺phosphors detected by a GaInAs infrared detector where the excitationspectrum was monitored at 910 nm.

FIG. 3 shows the decay curves of the afterglow (at 717 nm and 910 nm) ofexemplary LiGa₅O₈:0.001Cr³⁺ exposed to a 4-W 254-nm ultraviolet lamp for5 minutes.

FIG. 4 shows the x-ay diffraction pattern of exemplaryLiGa.₅O₈:0.001Cr³⁺ phosphors.

FIG. 5 shows the near infrared images of the afterglow of exemplaryLiGa₅O₈:Cr³⁺,R²⁺ phosphor disks after exposure to a 4-W 254-nmultraviolet lamp for 5 minutes. The afterglow can last more than 100hours.

FIG. 6 shows the excitation and emission of exemplaryLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ phosphors where the excitation spectrum wasmonitored at 717 nm.

FIG. 7 shows the excitation and emission of exemplaryLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ phosphors detected by a GaInAs infrareddetector where the excitation spectrum was monitored at 910 nm.

FIG. 8 shows the decay curves of the afterglow (at 717 nm and 910 nm) ofexemplary LiGa₅O₈:0.001Cr³⁺,0.01C a²⁺ exposed to a 4-W 254-nmultraviolet lamp for 5 minutes.

FIG. 9 shows the near infrared images of the afterglow of exemplaryLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ phosphor disks exposed to sunlight for 5minutes, 30 minutes, and 60 minutes. The afterglow can last more than100 hours.

FIG. 10 shows the near infrared images of the afterglow of exemplaryLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ phosphor disks with diameters of 15 mm, 40mm, and 55 mm exposed to solar radiation for 5 minutes on a cloudy day.

FIG. 11 shows the near infrared images of exemplaryLiGa₅O₈:0.001Cr³⁺,0.01C a²⁺ phosphor disks which were exposed tosunlight for 5 minutes and then were immersed into salt (NaCl) water.The phosphorescence of the disks in salt water can last more than 100hours.

FIG. 12 shows the near infrared images of exemplaryLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ phosphor disks activated by solar radiationin sunlight for 5 minutes when they were immersed in tap water and aNaCl-bleach-bicarbonate (NaHCO₃) aqueous solution.

FIG. 13 shows the decay curves of the afterglow (at 717 nm) of exemplaryLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ exposed for 5 minutes to ultraviolet light,sunlight (in sunny day), and diffused solar radiation (in rainy day).

FIG. 14 shows the decay curves of the afterglow (at 717 nm) of exemplaryLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ exposed for 5 minutes to ultraviolet lightand sunlight when they were immersed in tap water and salt water.

FIG. 15 shows the near infrared images of ONR and UGA logos drawn withexemplary LiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺-containing paint. The logos wereactivated by a 254 nm lamp for 5 minutes.

FIG. 16 shows the near infrared images of ONR and UGA logos drawn withexemplary LiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺-containing paint. The logos wereactivated by sunlight for 5 minutes.

FIG. 17 shows the excitation and emission of exemplaryLiAlGa₄O₈:0.001Cr³⁺,0.01C a²⁺ phosphors where the excitation spectrumwas monitored at 717 nm.

FIG. 18 shows the excitation and emission of exemplaryLiAlGa₄O₈:0.001Cr³⁺,0.01Ca²⁺ phosphors detected by a GaInAs infrareddetector where the excitation spectrum was monitored at 910 nm.

FIG. 19 shows the decay curves of the afterglow (at 717 nm and 910 nm)of exemplary LiAlGa₄O₈:0.001Cr³⁺,0.01Ca²⁺ exposed to a 4-W 254-nmultraviolet lamp for 5 minutes.

FIG. 20 shows the excitation and emission of exemplaryLiGa₄GeO_(8.5):0.001Cr³⁺,0.01Ca²⁺ phosphors where the excitationspectrum was monitored at 717 nm.

FIG. 21 shows the excitation and emission of exemplaryLiGa₄GeO_(8.5):0.001Cr³⁺,0.01Ca²⁺ phosphors detected by a GaInAsinfrared detector where the excitation spectrum was monitored at 910 nm.

FIG. 22 shows the decay curves of the afterglow (at 717 nm and 910 nm)of exemplary LiGa₄GeO_(8.5):0.001Cr³⁺,0.01Ca²⁺ exposed to a 4-W 254-nmultraviolet lamp for 5 minutes.

FIG. 23 shows the excitation and emission of exemplary LiGa₅O₈:0.001Ni²⁺phosphors detected by a GaInAs infrared detector where the excitationspectrum was monitored at 1258 nm.

FIG. 24 shows the decay curve of the afterglow (at 1258 nm) of exemplaryLiGa₅O₈:0.001Ni²⁺ exposed to a 4-W 254-nm wavelength ultraviolet lampfor 5 minutes.

FIG. 25 shows the excitation and emission of exemplaryLiGa₅O₈:0.001Cr³⁺,0.001Ni²⁺ phosphors where the excitation spectrum wasmonitored at 717 nm.

FIG. 26 shows the excitation and emission of exemplaryLiGa₅O₈:0.001Cr³⁺,0.001Ni²⁺ phosphors detected by a GaInAs infrareddetector where the excitation spectrum was monitored at 1258 nm.

FIG. 27 shows the decay curves of the afterglow (at 717 nm and 1258 nm)of exemplary LiGa₅O₈:0.001Cr³⁺,0.001Ni²⁺ exposed to a 4-W 254-nmultraviolet lamp for 5 minutes.

FIG. 28 shows the excitation and emission of exemplary NaGa₅O₈:0.001Cr³⁺phosphors where the excitation spectrum was monitored at 728 nm.

FIG. 29 shows the excitation and emission of exemplary NaGa₅O₈:0.001Cr³⁺phosphors detected by a GaInAs infrared detector where the excitationspectrum was monitored at 910 nm.

FIG. 30 shows the decay curves of the afterglow (at 728 nm and 910 nm)of exemplary NaGa₅O₈:0.001Cr³⁺ exposed to a 4-W 254-nm wavelengthultraviolet lamp for 5 minutes.

FIG. 31 shows the excitation and emission of exemplary KGa₅O₈:0.001Cr³⁺phosphors where the excitation spectrum was monitored at 717 nm.

FIG. 32 shows the excitation and emission of exemplary KGa₅O₈:0.001Cr³⁺phosphors detected by a GaInAs infrared detector where the excitationspectrum was monitored at 910 nm.

FIG. 33 shows the decay curves of the afterglow (at 717 nm and 910 nm)of exemplary KGa₅O₈:0.001Cr³⁺ exposed to a 4-W 254-nm wavelengthultraviolet lamp for 5 minutes.

FIG. 34 shows the excitation and emission of exemplary LiGaO₂:0.001Cr³⁺phosphors where the excitation spectrum was monitored at 717 nm.

FIG. 35 shows the excitation and emission of exemplary LiGaO₂:0.001Cr³⁺phosphors detected by a GaInAs infrared detector where the excitationspectrum was monitored at 1240 nm.

FIG. 36 shows the decay curves of the afterglow (at 717 nm and 1240 nm)of exemplary LiGaO₂:0.001Cr³⁺ exposed to a 4 W 254 nm wavelengthultraviolet lamp for 5 minutes.

FIG. 37 shows the x-ay diffraction pattern of exemplary LiGaO₂:0.001Cr³⁺sample.

FIG. 38 shows the excitation and emission of exemplaryLiGaO₂:0.001Cr³⁺,0.001Ni²⁺ phosphors where the excitation spectrum wasmonitored at 717 nm.

FIG. 39 shows the excitation and emission of exemplaryLiGaO₂:0.001Cr³⁺,0.001Ni²⁺ phosphors detected by a GaInAs infrareddetector where the excitation spectrum was monitored at 1258 nm.

FIG. 40 shows the decay curves of the afterglow (at 717 nm and 1258 nm)of exemplary LiGaO₂:0.001Cr³⁺,0.001Ni²⁺ exposed to a 4 W 254 nmwavelength ultraviolet lamp for 5 minutes.

FIG. 41 shows the scanning electron microscope image of exemplaryLiGa₅O₈:Cr³⁺ nanophosphors synthesized by a sol-gel method.

FIG. 42 shows the decay curve of the afterglow (at 717 nm) of exemplaryLiGa₅O₈:Cr³⁺ nanophosphors exposed to a 4-W 254 nm ultraviolet lamp for5 minutes.

FIG. 43 shows the images of afterglow of exemplary LiGa₅O₈:Cr³⁺nanoparticles exposed to a 4-W 254 nm ultraviolet lamp for 5 minutes.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

This disclosure relates to long-persistent near infrared phosphors.Phosphors disclosed herein are based on doping of an activator (anemitter) into a host matrix. In particular, the compositions includealkaline gallate phosphors doped with Cr³⁺ or Ni²⁺ ions with persistentinfrared phosphorescence as long as 200 hours. The wavelength of theemission peak can be 690 to 1100 nm (for Cr³⁺) or 1100 to 1550 nm (forNi²⁺). The intensity of the afterglow and persistent time were improvedby co-doping proper alkaline earth trapping ions.

The phosphors disclosed herein include an alkaline gallate matrixactivated with Cr³⁺ or Ni²⁺ and codoped with certain alkaline earthmetal ions or transition metal ions. The phosphors can be activated with0.01 mol % to 5 mol % (preferably 0.1 mol % to 1.0 mol %) of Cr³⁺ orNi²⁺ activators and codoped with 0 to 5 mol % (preferably 0 to 2.0 mol%) of at least one alkaline earth metal ion or Zn²⁺ ion. The activatorand dopant concentration are measured in terms of mol % relative to Ga.

In one embodiment, the phosphors disclosed herein include a materialhaving one or more of the following formulas: AGa₅O₈:xC, yR; andAGaO₂:xC, yR, wherein a portion of Ga may optionally be replaced with aGroup IIIA metal (e.g., B, Al, and/or In) and/or a Group IVA metal(e.g., Ge, Si, and/or Sn); and wherein each A is independently analkaline metal (e.g., Li, Na, and/or K); each C is independently Cr³⁺,Ni²⁺, or a combination thereof; each R is independently a Zn²⁺ ion, analkaline earth metal ion (e.g., Mg²⁺, Ca²⁺, Sr²⁺, and/or Ba²⁺), or acombination thereof; each x is independently 0.01 to 5 and representsmol % based on the total moles of Ga and any replacements thereof; andeach y is independently 0 to 5 and represents mol % based on the totalmoles of Ga and any replacements thereof.

Preferred phosphors disclosed herein are those in which A is Li.Preferred phosphor hosts disclosed herein are therefore LiGa₅O₈ andLiGaO₂.

Phosphors disclosed herein are preferably activated with Cr³⁺ or Ni²⁺and codoped with an alkaline earth metal ions or transition metal ions.The phosphors may be codoped with a single ion or a combination of suchions selected from the group of alkaline earth metal ions: Mg²⁺, Ca²⁺,Sr²⁺ and Ba²⁺ and the transition metal ion Zn²⁺. Codoping results inphosphors of improved brightness and persistence times.

Phosphors disclosed herein also include those in which Na⁺ or K⁺ issubstituted for Li⁺ in the matrix material and in which Ga³⁺ ispartially replaced with a Group IIIA metal ion (e.g., B³⁺, Al³⁺, orIn³⁺) or a Group IVA metal ion (e.g., Si⁴⁺, Ge⁴⁺ or Sn⁴⁺) in the matrix.These substitutions are believed to effect charge compensation.

Phosphor materials disclosed herein can preferably exhibit superiorphosphorescence intensity and long persistence of phosphorescence.Persistence of phosphorescence is estimated herein as persistence timewhich is the time after discontinuing irradiation that it takes forphosphorescence of a sample to decrease to the threshold of thesensitivity of a Generation III night vision goggles in the dark. Thetendency of the phosphorescence decay is also assessed by aFuoroLog3-2iHR320 spectrofluorometer by following the phosphorescenceintensity as a function of time. All the measurements and assessmentswere performed under identical conditions using the same detectionsystems. Materials disclosed herein can preferably exhibit persistencetime up to 200 hours or more. It is generally the case that phosphorshaving longer persistence times are more preferred.

The hosts disclosed herein include alkaline gallates AGa₅O₈ and AGaO₂,where A is an alkaline metal Li, Na, or K. Hosts where A is Li are morepreferred. The more preferred hosts are therefore lithium gallatesLiGa₅O₈ and LiGaO₂. A slight excess over the stoichiometric amount ofalkaline A (Li, Na and K) may be added to compensate for any A⁺ that maybe evaporated during sintering.

The activator employed in the phosphors disclosed herein includes Cr³⁺or Ni²⁺ or a combination of both. The Cr³⁺-activated phosphors havephosphorescent bands at 690-1100 nm. The adding of Ni²⁺ ion creates anemission band at 690-1100 nm. The concentration of the activator isprovided with an amount which is sufficient to produce a phosphor havinghigh phosphorescence intensity and long persistence time. The preferredconcentration of the activator in the phosphors disclosed herein is 0.1mol % to 1.0 mol %, which is measured in term of mol % relative to Ga.

This disclosure demonstrates that doping of an alkaline earth metal ion(e.g., Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) or Zn²⁺ ion into the host matrixdisclosed herein can result in phosphors having improved phosphorescencebrightness and persistence time. It is believed that these dopants cancreate proper trapping centers in the matrix, which can store excitationenergy and release gradually to the emitter. Preferred for the hostsdisclosed herein is doping with Ca²⁺, Sr²⁺ or Zn²⁺. The preferredconcentration of the dopant is 0 to 2.0 mol %, which is measured in termof mol % relative to Ga.

Phosphors disclosed herein also include those in which a portion of Ga³⁺in the host is replaced with a trivalent ion, such as Group IIIA metalions (R³⁺) B³⁺, Al³⁺ or In³⁺, or a tetravalent ion, such as Group IVAmetal ions (R⁴⁺) Si⁴⁺, Ge⁴⁺ or Sn⁴⁺. The more preferred trivalent ion isAl³⁺ and the more preferred tetravalent ion is Ge⁴⁺. The preferredR³⁺/Ga³⁺ or R⁴⁺/Ga³⁺ ratio is from 0.1 to 0.5. For Ga³⁺—R⁴⁺substitution, the doping level is designed to compensate the chargeeffects which are induced due to substitution Ga³⁺ by R⁴⁺.

This disclosure exemplifies phosphors in powder and ceramic formsprepared by combing the host, activator and dopant. The phosphorsdisclosed herein can be made by the following general solid-statereaction method, preferably providing particles that are typically 10micrometers or larger. The phosphor components are combined as indicatedin stoichiometric formulas. The raw materials are mixed and ground tofine powder followed by preferring at 800-1000° C. in air for 2-5 hours.The prefired material is again ground to fine powder. The prefiredpowder is preferably pressed into ceramic disks with diameters varyingfrom 15 mm to 70 mm. The powder or disks are then sintered at 1200-1400°C. for 2-6 hours in air.

The phosphors disclosed herein can also be made into nanoparticles by asol-gel method using the following procedure. A solution of appropriateamount of host nitrates [LiNO₃ and Ga(NO₃)₃.6H₂O], activator nitrate[Cr(NO₃)₃.9H₂O], ethanol, glycerol, and citric acid (as chelant) wasintimately stirred for 4 hours on a magnet stirrer. After gelation, thegel was heated at 60-80° C. to form dry gel followed by calcination at600-900° C. for 2-6 hours. The obtained nanoparticles have diameters of20-200 nm.

The phosphors disclosed herein can preferably be effectively activatedby a wide range of excitation sources including solar radiation(including diffuse light), direct sunlight, ultraviolet lamp,fluorescent lamp, and LED light.

The phosphors disclosed herein can preferably be effectively activatedby the above mentioned excitation sources in various medium including inair, tap water, salt water (same NaCl concentration as the sea water),bleach water, and bleach-salt-sodium bicarbonate (NaHCO₃) aqueoussolution. The samples excited in these aqueous medium preferably havesimilar excitation, emission, and persistence performance as thoseexcited in air under the same excitation source.

The phosphors disclosed herein can preferably be effectively activatedby solar radiation in various weathering conditions including in sunnyday, partly cloudy day, heavy cloudy day, rainy day, and heavy rainyday.

The phosphors disclosed herein can preferably be effectively activatedby solar radiation at any outdoor locations including in open area underany weathering conditions, in the shadow of trees, and in the shadow ofbuildings under any weathering conditions.

The phosphors disclosed herein can preferably be effectively activatedby solar radiation at anytime of the day, including the moment beforesunrise and after sunset, as long as the outside is visible.

The phosphors disclosed herein can preferably be quickly charged by theabove mentioned excitation sources. For example, the energy storedduring less than 1 minute of excitation by solar radiation andultraviolet lamp can sustain up to 200 hours of continuous near infraredemission. Such charging (light absorption-light emission) can preferablybe recycled indefinitely.

The phosphors disclosed herein are preferably extremely chemicallystable in outdoor application environments including such severeenvironments as in water, sea water, swimming pool water, and sawingwater. The samples immersed in the above water for six months canexhibit the same excitation, emission, and persistence performance asthe fresh ones.

The powder phosphors disclosed herein can be incorporated into variouswater-based and oil-based paints to form near infrared luminescentpaints or inks. This can preferably endow the paints or inks with thefunction of near infrared luminescence in the dark. The water-basedpaints are preferably regular indoor-used wall paints. The quantity ofthe phosphor powder added to the water-based paint is typically 10 wt. %to 50 wt. %, preferably 20 wt. % to 30 wt. %. The oil-based paints aremainly transparent or colorless resins and varnish. The preferred resinsinclude epoxy resin, polyurethane resin, polyester resin, acrylic acidresin, and hydroxyl acrylic acid resin. The preferred varnish includesamino varnish, acrylic polyurethane coating, and transparent alkydcoating. The quantity of the phosphor powder added to the resins orvarnish is typically 10 wt. % to 50 wt. %, preferably 20 wt. % to 30 wt.%.

In making the oil-based near infrared illuminating paints, organicsolvent and auxiliary agents are usually added to improve the paint'sviscosity and fluidity. The organic solvents can include monohydricalcohols (such as ethanol, methanol, and isopropyl alcohol) and glycols(such as ethylene glycol and propylene glycol). The agents are mainlydispersing agents (such as methyl xylene solution) and sediment-freeagents (such as silica fine powders).

For small amount of usage, the phosphor powders, organic solvent, andagents can be mixed manually using a glass rod. For large amount ofusage, a high-speed mixer can be used to mix these components. Thepreferred mixing procedure using a mixer is as follows. A certain amountof resin (or varnish), organic solvent, and dispersing agent are addedinto a container before stirring. The mixer is then turned on and thesediment-free agent and phosphor powders are added slowly. The mixercontinues to spin until the phosphor powders are uniformly dispersed inthe paint.

The paints and inks can be applied to any surfaces including rocks,building walls, trees, highways, runways, planes, ships, vehicles,machines, clothes, helmets, weapons, boards, control panels, etc.

The phosphors disclosed herein can also preferably be incorporated intotransparent silicone rubbers and plastics, endowing the rubbers andplastics with the function of near infrared luminescence in the dark.Significantly, the near infrared luminescent silicone rubbers canexhibit a good degree of deformability without cracking.

The phosphors disclosed herein can preferably be used as invisible (bynaked eye) illumination source and identification markers in the darkfor military, security, and forensic related applications. For example,the markers (combat ID) made from the phosphors disclosed herein can beattached onto the soldier's cloth or helmet, which can be recognized andmonitored by a night vision goggles for tracking and locating purposes.The phosphors can be used either as solid ceramic or luminescent paintsor inks.

The phosphors disclosed herein can also be used as identificationmarkers in the dark for rescue purposes. For example, a wreckage shippainted with the phosphors-disclosed herein can be easily found with anight vision device from a rescue helicopter. Another example is minerescue. The miners with their helmets and cloths painted with thephosphors disclosed herein can be easily searched with a night visiongoggles.

Due to their superior capability in absorbing solar radiation and theirability in converting the solar energy into near infrared photons, thephosphors disclosed herein may also be used to improve the efficiency,effectiveness and productivity of the widely deployed Si solar cells.This is because the near infrared photons (from 690 nm to 1100 nm forCr³⁺ activated phosphors, which corresponds to 1.1-1.8 eV in electronvolts) emitted by the phosphors correspond to an optimum spectralresponse of the Si solar cells (band gap is approximately 1.1 eV). Thepossible routes include: (1) making cover glass that contains thephosphors disclosed herein; and (2) coating the cover glass and thesilicon cell panels with the phosphors disclosed herein by sputteringcoating.

The nanoparticles disclosed herein may be used as an optical probe forin vivo bio-imaging. Due to the long afterglow, the probes can beexcited before injection. This can avoid the tissue autofluorescenceunder external illumination and thus can remove the background noiseoriginating from the in situ excitation. In addition, the skin andtissues are transparent to near infrared light, allowing deep tissueimaging.

The following examples are offered to further illustrate variousspecific embodiments and techniques of the present invention. It shouldbe understood, however, that many variations and modificationsunderstood by those of ordinary skill in the art may be made whileremaining within the scope of the present invention. Therefore, thescope of the invention is not intended to be limited by the followingexamples.

EXAMPLES Example 1 Method of Preparation of Near Infrared Phosphors withHost Materials AGa₅O₈ and AGaO₂ (where A is Li or Na or K)

Phosphor components are mixed according to the molar proportions in thefollowing general recipes:

For AGa₅O₈: 1.10 A₂CO₃+5.00 Ga₂O₃+xCr₂O₃ (and/or NiO)+yRO

For AGaO₂: 1.10 A₂CO₃+1.00 Ga₂O₃+xCr₂O₃ (and/or NiO)+yRO

where, preferably x=0.001 to 0.05 relative to Ga₂O₃ and more preferablyx=0.001 to 0.01; y is a number ranging from 0 to 0.05 and preferably y=0to 0.02; A is Li or Na or K. A slight excess over the stoichiometricamount of alkaline A (Li, Na and K) is added to compensate for any A⁺that may be evaporated during sintering; RO is alkaline earth metaloxide (e.g., MgO, CaO, SrO, or BaO) or ZnO.

The mixture of components is milled or ground to form a homogeneous finepowder for prefixing. The mixed powder is then prefired at 900° C. inair for 2 hours. The pre-fired material is again ground to fine powdersuitable for sintering. The prefired powder is preferably pressed intoceramic disks with diameters varying from 15 mm to 75 mm, preferably thediameter is 15 mm. The powder or disks are then sintered at 1300° C. inair for 2-6 hours. The resulting materials exhibit phosphor propertiesas described herein.

Example 2 Preparation and Characterization of LiGa₅O₈:Cr³⁺ Phosphors

LiGa₅O₈:0.001Cr³⁺ phosphor was prepared by the general method of Example1 mixing the components in the following molar proportions:

1.10 Li₂CO₃+5.00 Ga₂O₃+0.005 Cr₂O₃

A slight excess over the stoichiometric amount of Li₂CO₃ was added tocompensate for any Li⁺ that may have evaporated during sintering.

FIG. 1 presents the excitation and emission of LiGa₅O₈:0.001Cr³⁺phosphors where the excitation spectrum was monitored at 717 nm.

FIG. 2 shows the excitation and emission of LiGa₅O₈:0.001Cr³⁺ phosphorsdetected by a GaInAs infrared detector where the excitation spectrum wasmonitored at 910 nm.

FIG. 3 shows the decay curves of the afterglow of LiGa₅O₈:0.001Cr³⁺exposed to a 4 W 254 nm wavelength ultraviolet lamp for 5 minutes. Thedecays were monitored at 717 nm and 910 nm emissions.

FIG. 4 is the x-ay diffraction pattern of the LiGa₅O₈:0.001Cr³⁺ sample.

Example 3 Preparation and Characterization of LiGa₅O₈:Cr³⁺,R²⁺ Phosphors

The methods and phosphors disclosed herein are specifically exemplifiedby preparation of LiGa₅O₈:Cr³⁺,R²⁺ (Cr³⁺ and R²⁺-co-doped lithiumgallate) phosphors.

LiGa₅O₈:0.001Cr³⁺,0.01R²⁺ phosphor was prepared by the general method ofExample 1 mixing the components in the following molar proportions:

1.10 Li₂CO₃+5.00 Ga₂O₃+0.005 Cr₂O₃+0.1 RO

where RO is an oxide selected from MgO, CaO, BaO, SrO, and ZnO. A slightexcess over the stoichiometric amount of Li₂CO₃ was added to compensatefor any Li⁺ that may have evaporated during sintering. The opticalmeasurements were mainly carried out on the disk samples.

FIG. 5( a) is a digital image of eleven (11) LiGa₅O₈:Cr³⁺,R²⁺ and one(1) ZnGa₂O₄:Cr³⁺ (left top corner) phosphor disks (diameter: 15 mm)before excitation. The dopants R²⁺ include Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, andZn²⁺. The sintering durations were 2 hours, 4 hours, and 6 hours. After5 minutes excitation with a 254 nm ultraviolet lamp, theseLiGa₅O₈:Cr³⁺,R²⁺ phosphor disks emit near infrared afterglow that canlast more than 100 hours. FIG. 5( b) is the digital images showing thechange of afterglow brightness with lasting time. The images were takenby a digital camera via a Generation III night vision monocular in adark room. The number at the left top corner of each image is the timeafter which the image was taken. After 72 hours afterglow, the nearinfrared emission could still be clearly observed by the night visionmonocular and captured by the digital camera. These images clearly showthat co-doping of Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, or Zn²⁺ improves thephosphorescence brightness and persistent times. The best improvementwas obtained with Ca²⁺co-doping. The second best co-dopant is Mg²⁺ andthe third best co-dopant is Zn²⁺.

The following descriptions in Example 3 focus onLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺.

FIG. 6 presents the excitation and emission ofLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ phosphors where the excitation spectrum wasmonitored at 717 nm.

FIG. 7 shows the excitation and emission of LiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺phosphors detected by a GaInAs infrared detector where the excitationspectrum was monitored at 910 nm.

FIG. 8 shows the decay curves of the afterglow ofLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ exposed to a 4 W 254 nm wavelengthultraviolet lamp for 5 minutes. The decays were monitored at 717 nm and910 nm emissions.

FIG. 9 shows the images of the afterglow of six (6)LiGa₅O₈:0.001Cr³⁺,0.01C a²⁺ phosphor disks (diameter: 15 mm) exposed tosunlight for 5 minutes, 30 minutes, and 60 minutes. The weathercondition is fair sky/partly cloudy. The images were taken by a digitalcamera via a Generation III night vision monocular in a dark room. Thenumber at the bottom right corner of each image is the time after whichthe image was taken. After 138 hours afterglow, the near infraredemission could still be clearly observed by the night vision monocularand captured by the digital camera. These images clearly show that thesamples after 5 minutes, 30 minutes, and 60 minutes sunlight excitationhave similar phosphorescence brightness and persistence times.

FIG. 10 shows the images of afterglow of three (3)LiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ phosphor disks with diameters of 15 mm, 40mm, and 55 mm exposed to solar radiation for 5 minutes. The weathercondition is cloudy.

The images were taken by a digital camera via a Generation III nightvision monocular in a dark room. The number at the right top corner ofeach image is the time after which the image was taken. After 72 hoursafterglow, the near infrared emission can Still be clearly observed bythe night vision monocular and captured by the digital camera. Theseimages clearly show that the samples can be effectively excited by solarradiation even without direct sunlight and that the big and smallsamples have the same phosphorescence brightness and persistence times.

FIG. 11 shows the images of three (3) LiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺phosphor disks (diameter: 15 mm) which were exposed to sunlight for 5minutes and then (5 minutes later) were immersed into salt (NaCl) water.The images were taken by a digital camera via a Generation III nightvision monocular in a dark room. The concentration of NaCl in the saltwater is 3.5%, which is the same as the salinity of the seawater. Thephosphorescence of the disks in the salt water can be clearly seen after100 hours of emission via a night vision monocular. No apparentcorrosion was observed after 6 months of immersion in the salt water.The samples immersed in the salt water for six months exhibit the sameexcitation, emission, and persistence performance as the fresh ones.

FIG. 12 shows that the LiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ phosphor disks(diameter: 15 mm) can be effectively activated by solar radiation whenthey were immersed in tap water (FIG. 12 a) or a NaCl-bleach-bicarbonate(NaHCO₃) aqueous solution (FIG. 12 b). The exposure time under sunlightwas 5 minutes. The aqueous solution was made by adding 20 drops ofbleach, 5 grams of NaCl and 3 grams of NaHCO₃ into 75 ml tap water. Tokeep the solution in a basic pHs and oxidizing conditions, every week 20drops of bleach and 1 gram of NaHCO₃ were added to compensate theevaporation loss. Every week, the samples together with the solutionwere taken out for recharging by sunlight for 5 minutes and thephosphorescence was imaged by a digital camera via a Generation IIInight vision monocular in a dark room. No apparent corrosion wasobserved after 4 months of immersion in the NaCl-bleach-NaHCO₃ aqueoussolution. The samples immersed in the solution for four months exhibitedthe same excitation, emission, and persistence performance as the freshones.

FIG. 13 presents the decay curves of the afterglow ofLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ exposed to different excitation conditions:(1) to a 4-W 254-nm wavelength ultraviolet lamp for 5 minutes; (2) todirect sunlight for 5 minutes in a sunny day; and (3) to solar radiation(diffused light) for 5 minutes in a rainy day. The decays were monitoredat 717 nm. Note that for ultraviolet lamp excitation, the measurementstarted immediately after the lamp was turned off. For solar radiationexcitations, it took 2 minutes to bring the samples to the spectrometerfor measurements. This experiment clearly shows that theLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ phosphors can be efficiently activated bysolar radiation even in rainy days.

FIG. 14 shows the decay curves of the afterglow ofLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ exposed to different excitation sources andin different medium: (1) to a 4-W 254-nm wavelength ultraviolet lamp for5 minutes in tap water and salt water; and (2) to direct sunlight for 5minutes in tap water and salt water. The samples were immersed in waterfor both excitation and emission measurements. The NaCl concentration inthe salt water is 3.5%. Note that for ultraviolet lamp excitation, themeasurement started immediately after the lamp was turned off. For solarradiation excitations, it took 2 minutes to bring the samples to thespectrometer for measurements. These experiments clearly show that theLiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ phosphors can be efficiently activated byboth ultraviolet light and solar radiation even though they are immersedin tap water and salt water.

FIG. 15 shows the images of the logos of the Office of Naval Research(ONR) and the University of Georgia (UGA) written with the paint madefrom the LiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ phosphor powders with acrylicpolyurethane varnish. The logos were exposed to a 254 nm ultravioletlamp for 5 minutes. The quantity of the phosphor powders in the varnishwas 30 wt. %. The images were taken by a digital camera via a GenerationIII night vision monocular in a dark room. The logos can be clearly seenafter 100 hours of emission via a night vision monocular.

FIG. 16 shows the images of the logos of the Office of Naval Research(ONR) and the University of Georgia (UGA) written with the paint madefrom the LiGa₅O₈:0.001Cr³⁺,0.01Ca²⁺ phosphor powders with acrylicpolyurethane varnish. The logos were exposed to sunlight for 5 minutes.The quantity of the phosphor powders in the varnish was 30 wt. %. Theimages were taken by a digital camera via a Generation III night visionmonocular in a dark room. The logos can be clearly seen after 100 hoursof emission via a night vision monocular

Example 4 Preparation and Characterization of LiAlGa₄O₈:Cr³⁺,Ca²⁺Phosphors

LiAlGa₄O₈:0.001Cr³¹,0.01Ca²⁺ phosphor was prepared by the general methodof Example 1 mixing the components in the following molar proportions:

1.10 Li₂CO₃+4.00 Ga₂O₃+1.00 Al₂O₃+0.005 Cr₂O₃+0.1 CaO

A slight excess over the stoichiometric amount of Li₂CO₃ was added tocompensate for any Li⁺ that may have evaporated during sintering.

FIG. 17 presents the excitation and emission ofLiAlGa₄O₈:0.001Cr³⁺,0.01Ca²⁺ phosphors where the excitation spectrum wasmonitored at 717 nm.

FIG. 18 shows the excitation and emission ofLiAlGa₄O₈:0.001Cr³⁺,0.01Ca²⁺ phosphors detected by a GaInAs infrareddetector where the excitation spectrum was monitored at 910 nm.

FIG. 19 shows the decay curves of the afterglow ofLiAlGa₄O₈:0.001Cr³⁺,0.01Ca²⁺ exposed to a 4-W 254-nm ultraviolet lampfor 5 minutes. The decays were monitored at 717 nm and 910 nm emissions.

Example 5 Preparation and Characterization of Li₂Ga₈Ge₂O₁₇:Cr³⁺,Ca²⁺Phosphors

Li₂Ga₈Ge₂O₁₇:0.001Cr³⁺,0.01Ca²⁺ phosphor was prepared by the generalmethod of Example 1 mixing the components in the following molarproportions:

1.10 Li₂CO₃+4.00 Ga₂O₃+1.00 GeO₂+0.005 Cr₂O₃+0.1 CaO

A slight excess over the stoichiometric amount of Li₂CO₃ was added tocompensate for any Li⁺ that may have evaporated during sintering.

FIG. 20 presents the excitation and emission ofLi₂Ga₈Ge₂O₁₇:0.001Cr³⁺,0.01Ca²⁺ phosphors where the excitation spectrumwas monitored at 717 nm.

FIG. 21 shows the excitation and emission ofLi₂Ga₈Ge₂O₁₇:0.001Cr³⁺,0.01Ca²⁺ phosphors detected by a GaInAs infrareddetector where the excitation spectrum was monitored at 910 nm.

FIG. 22 shows the decay curves of the afterglow ofLi₂Ga₈Ge₂O₁₇:0.001Cr³⁺,0.01Ca²⁺ exposed to a 4-W 254-nm ultraviolet lampfor 5 minutes. The decays were monitored at 717 nm and 910 nm emissions.

Example 6 Preparation and Characterization of LiGa₅O₈:Ni²⁺ Phosphors

LiGa₅O₈:0.001Ni²⁺ phosphor was prepared by the general method of Example1 mixing the components in the following molar proportions:

1.10 Li₂CO₃+5.00 Ga₂O₃+0.01 NiO

A slight excess over the stoichiometric amount of Li₂CO₃ was added tocompensatefor any Li⁺ that may have evaporated during sintering.

FIG. 23 presents the excitation and emission of LiGa₅O₈:0.001Ni²⁺phosphors detected by a GaInAs infrared detector where the excitationspectrum was monitored at 1258 nm.

FIG. 24 shows the decay curve of the afterglow of LiGa₅O₈:0.001Ni²⁺exposed to a 4-W 254-nm ultraviolet lamp for 5 minutes. The decays weremonitored at 1258 nm emission.

Example 7 Preparation and Characterization of LiGa₅O₈:Cr³⁺,Ni²⁺Phosphors

LiGa₅O₈:0.001Cr³⁺,0.001Ni²⁺ phosphor was prepared by the general methodof Example 1 mixing the components in the following molar proportions:

1.10 Li₂CO₃+5.00 Ga₂O₃0.005 Cr₂O₃0.01 NiO

A slight excess over the stoichiometric amount of Li₂CO₃ was added tocompensate for any Li⁺ that may have evaporated during sintering.

FIG. 25 presents the excitation and emission ofLiGa₅O₈:0.001Cr³⁺,0.001Ni²⁺ phosphors where the excitation spectrum wasmonitored at 717 nm. The 717 nm emission band is from the characteristictransition of Cr³⁺ ion.

FIG. 26 shows the excitation and emission of LiGa₅O₈:0.001Cr³⁺,0.001Ni²⁺phosphors detected by a GaInAs infrared detector where the excitationspectrum was monitored at 1258 nm. The 1258 nm emission band is from thecharacteristic transition of Ni²⁺ ion.

FIG. 27 shows the decay curves of the afterglow ofLiGa₅O₈:0.001Cr³⁺,0.001Ni²⁺ exposed to a 4-W 254-nm ultraviolet lamp for5 minutes. The decays were monitored at 717 nm and 1258 nm emissions.

Example 8 Preparation and Characterization of NaGa₅O₈:Cr³⁺ Phosphors

NaGa₅O₈:0.001Cr³⁺ phosphor was prepared by the general method of Example1 except that the final sintering temperature was 1250° C. (to avoidmelting of the material). The source components were mixed in thefollowing molar proportions:

1.10 NaHCO₃+5.00 Ga₂O₃+0.005 Cr₂O₃

A slight excess over the stoichiometric amount of NaHCO₃ was added tocompensate for any Na⁺ that may have evaporated during sintering.

FIG. 28 presents the excitation and emission of NaGa₅O₈:0.001Cr³⁺phosphors where the excitation spectrum was monitored at 728 nm. Theemission band includes two peaks: 704 nm and 728 nm.

FIG. 29 shows the excitation and emission of NaGa₅O₈:0.001Cr³⁺ phosphorsdetected by a GaInAs infrared detector where the excitation spectrum wasmonitored at 910 nm.

FIG. 30 shows the decay curves of the afterglow of NaGa₅O₈:0.001Cr³⁺exposed to a 4-W 254-nm ultraviolet lamp for 5 minutes. The decays weremonitored at 728 nm and 910 nm emissions.

Example 9 Preparation and Characterization of KGa₅O₈:Cr³⁺ Phosphors

KGa₅O₈:0.001Cr³⁺ phosphor was prepared by the general method of Example1 except that the final sintering temperature was 1275° C. (to avoidmelting of the material). The source components were mixed in thefollowing molar proportions:

1.10 KHCO₃+5.00 Ga₂O₃+0.005 Cr₂O₃

A slight excess over the stoichiometric amount of KHCO₃ was added tocompensate for any K⁺ that may have evaporated during sintering.

FIG. 31 presents the excitation and emission of KGa₅O₈:0.001Cr³⁺phosphors where the excitation spectrum was monitored at 717 nm. Theemission band includes two peaks: 708 nm and 717 nm.

FIG. 32 shows the excitation and emission of KGa₅O₈:0.001Cr³⁺ phosphorsdetected by a GaInAs infrared detector where the excitation spectrum wasmonitored at 910 nm.

FIG. 33 shows the decay curves of the afterglow of KGa₅O₈:0.001Cr³⁺exposed to a 4-W 254-nm ultraviolet lamp for 5 minutes. The decays weremonitored at 717 nm and 910 nm emissions.

Example 10 Preparation and Characterization of LiGaO₂:Cr³⁺ Phosphors

LiGaO₂:0.001Cr³⁺ phosphor was prepared by the general method of Example1 mixing the components in the following molar proportions:

1.10 Li₂CO₃+1.00 Ga₂O₃+0.001Cr₂O₃

A slight excess over the stoichiometric amount of Li₂CO₃ was added tocompensate for any Li⁺ that may have evaporated during sintering

FIG. 34 presents the excitation and emission of LiGaO₂:0.001Cr³⁺phosphors where the excitation spectrum was monitored at 717 nm.

FIG. 35 shows the excitation and emission of LiGaO₂:0.001Cr⁺ phosphorsdetected by a GaInAs infrared detector where the excitation spectrum wasmonitored at 1240 nm. The 1240 nm band is weak and broad.

FIG. 36 shows decay curves of the afterglow of LiGaO₂:0.001Cr³⁺ exposedto a 4 W 254 nm ultraviolet lamp for 5 minutes. The decays weremonitored at 717 nm and 1240 nm emissions.

FIG. 37 is the x-ay diffraction pattern of the LiGaO₂:0.001Cr³⁺ sample.

Example 11 Preparation and Characterization of LiGaO₂:Cr³⁺,Ni²⁺Phosphors

LiGa)₂:0.001Cr³⁺,0.001Ni²⁺ phosphor was prepared by the general methodof Example 1 mixing the components in the following molar proportions:

1.10 Li₂CO₃+1.00 Ga₂O₃+0.001 Cr₂O₃+0.002 NiO

A slight excess over the stoichiometric amount of Li₂CO₃ was added tocompensate for any Li⁺ that may have evaporated during sintering.

FIG. 38 presents the excitation and emission ofLiGaO₂:0.001Cr³⁺,0.001Ni²⁺ phosphors where the excitation spectrum wasmonitored at 717 nm. The 717 nm emission band is from the characteristictransition of Cr³⁺ ion.

FIG. 39 shows the excitation and emission of LiGaO₂:0.001Cr³⁺,0.001Ni²⁺phosphors detected by a GaInAs infrared detector where the excitationspectrum was monitored at 1258 nm. The 1258 nm emission band is from thecharacteristic transition of Ni²⁺ ion.

FIG. 40 shows the decay curves of the afterglow ofLiGaO₂:0.001Cr³⁺,0.001Ni²⁺ exposed to a 4 W 254 nm ultraviolet lamp for5 minutes. The decays were monitored at 717 nm and 1258 nm emissions.

Example 12 Preparation and Characterization of LiGa₅O₈:Cr³⁺Nanophosphors

The LiGa₅O₈:Cr³⁺ nanophosphors disclosed herein was synthesized by asol-gel method using the following procedure. A solution of appropriateamount of host nitrates [LiNO₃ and Ga(NO₃)₃.6H₂O], activator nitrate[Cr(NO₃)₃. 9H₂O], ethanol, glycerol, and citric acid (as chelant) wasintimately stirred for 4 hours on a magnet stirrer. After gelation, thegel was heated at 60-80° C. to form dry gel followed by calcination at600-900° C. for 2-6 hours.

FIG. 41 is the scanning electron microscope image of LiGa₅O₈:Cr³⁺nanophosphors synthesized by a sol-gel method. The nanoparticles havediameters in the range of 20-200 nm.

FIG. 42 shows the decay curve of the afterglow of LiGa₅O₈:Cr³⁺nanophosphors exposed to a 4-W 254 nm ultraviolet lamp for 5 minutes.The decays were monitored at 717 nm.

FIG. 43 shows the images of afterglow of LiGa₅O₈:Cr³⁺ nanoparticlesexposed to a 4-W 254 nm ultraviolet lamp for 5 minutes. Thenanoparticles were placed at the bottom of a glass vial. The images weretaken by a digital camera via a Generation III night vision monocular ina dark room. The time at the up right corner of each image is the timeafter which the image was taken. After 120 hours afterglow, the nearinfrared emission from the nanoparticles can still be clearly observedby the night vision monocular and captured by the digital camera.

The complete disclosures of the patents, patent documents, andpublications cited herein are incorporated by reference in theirentirety as if each were individually incorporated. Variousmodifications and alterations to this invention will become apparent tothose skilled in the art without departing from the scope and spirit ofthis invention. It should be understood that this invention is notintended to be unduly limited by the illustrative embodiments andexamples set forth herein and that such examples and embodiments arepresented by way of example only with the scope of the inventionintended to be limited only by the claims set forth herein as follows.

1. A phosphor comprising a material having one or more of the followingformulas:AGa₅O₈:xC, yR; andAGaO₂: xC, yR wherein a portion of Ga may optionally be replaced with aGroup IIIA metal and/or a Group IVA metal; and wherein each A isindependently an alkaline metal; each C is independently Cr³⁺, Ni²⁺, ora combination thereof; each R is independently a Zn²⁺ ion, an alkalineearth metal ion, or a combination thereof; each x is independently 0.01to 5 and represents mol % based on the total moles of Ga and anyreplacements thereof; and each y is independently 0 to 5 and representsmol % based on the total moles of Ga and any replacements thereof. 2.The phosphor of claim 1 wherein each A is independently selected fromthe group consisting of Li, Na, K, and combinations thereof.
 3. Thephosphor of claim 1 wherein each x is independently 0.05 to 0.5 andrepresents mol % based on the total moles of Ga and any replacementsthereof.
 4. The phosphor of claim 1 wherein each y is independently 0.1to 2 and represents mol % based on the total moles of Ga and anyreplacements thereof.
 5. The phosphor of claim 1 wherein each R isindependently selected from the group consisting of Mg²⁺, Ca²⁺, Sr²⁺,Ba²⁺, Zn²⁺, and combinations thereof.
 6. The phosphor of claim 1 whereineach C is Cr³⁺.
 7. The phosphor of claim 6 having emission band peaks at690 to 1100 nm.
 8. The phosphor of claim 1 wherein each C is Ni²⁺. 9.The phosphor of claim 8 having emission band peaks at 1100 to 1550 nm.10. The phosphor of claim 1 wherein each C is a combination of Cr³⁺ andNi²⁺.
 11. The phosphor of claim 10 having emission band peaks at 690 to1100 nm and 1100 to 1550 nm.
 12. The phosphor of claim 1 wherein aportion of Ga is substituted with a Group IIIA metal.
 13. The phosphorof claim 12 wherein the Group IIIA metal is selected from the groupconsisting of B, Al, In, and combinations thereof.
 14. The phosphor ofclaim 1 wherein a portion of Ga is substituted with a Group IVA metal.15. The phosphor of claim 14 wherein the Group IVA metal is selectedfrom the group consisting of Ge, Si, Sn, and combinations thereof 16-21.(canceled)
 22. The phosphor of claim 1 wherein an emission persists forup to 200 hours after excitation.
 23. The phosphor of claim 1 whereinthe phosphor is chemically stable in one or more of air, tap water, saltwater, seawater, bleach water, and bleach-salt-sodium bicarbonate(NaHCO₃) aqueous solution. 24-32. (canceled)
 33. A luminescent paintcomprising a phosphor according to claim
 1. 34. A method of activating aphosphor comprising: providing a phosphor according to claim 1; andexposing the phosphor to one or more of solar radiation, an ultravioletlamp, a fluorescent lamp, and a light emitting diode (LED) light.
 35. Anactivated phosphor comprising a phosphor according to claim 1 that hasbeen exposed to one or more of solar radiation, an ultraviolet lamp, afluorescent lamp, and a light emitting diode (LED) light, and whereinthe activated phosphor has an emission that persists for up to 200 hoursafter excitation.